Adenosine is an endogenous purine nucleoside that depresses the sinoatrial node, blocks atrioventricular conduction, inhibits the pacemaker activity of the His-Purkinje system and attenuates the effects of catecholamines. Since adenosine is produced and released by myocardial cells during ischemia and hypoxia, it may be a reversible factor in the etiology or perpetuation of bradyasystole. Aminophylline is a competitive antagonist of adenosine. The use of aminophylline for bradycardia and heart block has been described, and a number of anecdotal reports and small studies have been published on the use of aminophylline in cardiac arrest. We undertook this study to evaluate the effect of aminophylline during cardiopulmonary resuscitation (CPR) of patients with out-of-hospital bradyasystolic cardiac arrest unresponsive to initial therapy.